Use of fluorescent labels in biodetection has revolutionized how research scientists detect, analyze, and quantify biological materials and systems. Fluorescent labels emit light upon excitation by an external energy source. Fluorescent labels have a number of applications in biology including, for example, fluorescence flow cytometry, fluorescence-activated cell sorting, fluorescence microscopy, and fluorescence immunoassays. Semiconductor nanocrystals, known as Quantum Dots (or “QDs”), have emerged over the past twenty years as an interesting class of nanomaterials with optical properties that have potential applications in a variety of fields, including biological imaging and detection, (Alivisatos 1996 Science, Vol. 271, no. 5251, pp, 933-937; Murray, et al 2000 Annu. Rev. Mat. Sci., 30:545-310.) QDs have been used as fluorescent tags and have shown unique properties in comparison to common fluorophores for applications in biological imaging and diagnostics. (Michalet, et al. 2005 Science: Vol. 307. no. 5709, pp. 538-544.) A common type of semiconductor QDs is CdSe, which is typically grown with a ZnS shell, for example, as a CdSe—ZnS core-shell structure. (Hines, et al. 1996 J. Phys. Chem. 100 (2), pp 468-471.) To protect the core and to increase the quantum yield of the QDs (i.e., the fluorescence emission intensity), QDs have been capped with a shell that has a larger band gap than the core, such as in CdSe—ZnS core-shell structures.
Despite the progress in biodetection in recent years, significant constraints exist that limit the applicability of fluorophore labels. Existing systems suffer from significant shortcomings, such as lack of sufficient sensitivity, complexity of system or operation, lengthy data collection and analysis, insufficient selectivity or specificity, requirement of special equipment or set up, and high cost of instruments and/or operation.
The toxicity of the Cd2+ ions is a limiting factor for in vivo applications of Cd-containing QDs and a concern for commercial manufacturing of Cd-based nanocrystals. (Kirchner, et al. 2005 Nano Letters 5 (2):331-338.) As a result, “Cd-free” QDs based on ZnSe have attracted attention, because they do not involve a heavy metal ZnSe-QDs can be grown by a variety of techniques and exhibit size-tunable emission wavelength over a region m the UV-blue part of the spectrum (370-460 nm), which is narrower when compared to the spectral range of CdSe QDs (470-650 nm). (Leppert, et al 1997 Philosophical Magazine Letters 75 (1):29-33; Hines, et al. 1998 J. Phys. Chem. B 102 (19):3655-3657; Quinlan, et al. 2000 Langmuir 16 (8):4049-4051; Sarigiannis, et al. 2002 Applied Physics Letters 80 (21):4024-4026; Karanikolos, et al. 2004 Langmuir 20 (3):550-553; Karanikolos, et al. 2005 Nanotechnology 16 (10): 2372-2389; Karanikolos, et al. 2006 Nanotechnolgy 17 (13):3121-3128; Pradhan, et al. 2005 J. Am. Chem. Soc. 127 (50):17586-17587.)
Doping of ZnSe QDs with transition metals ions, such as Mn2+ enables timing of their emission to longer wavelengths, thus providing the ability to develop multi-color probes for biological tagging applications and magneto-optical materials for spintronics. (Pradhan, et al. 2007 J. Am. Chem. Soc. 129 (11):3339-3347; Pradhan, et al. 2007 Nano Letters 7 (2):312-317; Erwin, et al. 2005 Nature 436 (7047):91-94; Norris, et al. 2001 Nano Letters 1 (1):3-7.)
There is therefore an unmet need for improved biodetection sensors, systems, and methods that allow efficient, effective and low cost fluorescent imaging and diagnostic applications.